1. Field of the Invention
The present invention relates to image formation apparatus and method for reading an image on an original, forming the read original image on a sheet of paper (referred as sheet hereinafter), and performing a postprocess to the sheet on which the image has been formed.
2. Related Background Art
Conventionally, there is known an image formation system in which an image formation apparatus for reading an image of an original and forming the read image on a sheet mounts with a postprocess device for performing a binding process, a folding process, a sorting process and the like to the sheets on which the images have been formed, whereby it is possible to reduce work such as sheet binding, sheet folding, sheet rearranging and the like.
As the postprocess device, there is a device capable of performing a series of processes to fold and bind the sheets and finally perform book binding. Such the postprocess device capable of performing a series of processes has a saddle-stitching mechanism which performs a process to fold the sheets in half and then performs a process to staple the folded sheets together. The saddle-stitching mechanism presses the predetermined portion of the sheet against a roller member to form the fold or crease on the sheet.
However, a friction coefficient between the roller member and the predetermined portion of the sheet is significantly different between a case where the roller member is in contact with the predetermined portion of the sheet on which the image has been formed and a case where the roller member is in contact with the predetermined portion of the sheet on which any image is not formed. For this reason, since the roller member may sometimes slip off the predetermined portion in either of these cases, the fold positions may be sometimes misregistered in either of these cases. As a result, since it is impossible to form the fold at the optimum position because of such misregistration of the sheets, it may be impossible to accurately perform the book binding process.
In order to avoid a situation wherein the fold positions are misregistered between the case where the image has been formed at the predetermined portion of the sheet and the case where any image is not formed at this position, it is considered reducing a variation of the friction coefficient between the roller member and the predetermined portion of the sheet as small as possible for each of these cases by changing materials of the roller member and the sheet. However, such a method cannot cope with variation of the friction coefficient due to variations of temperature and humidity of an atmosphere in which the sheets are placed. Thus, the fold positions of the sheets are similarly misregistered because of the variation of the friction coefficient between the roller member and the predetermined portion with which the roller member contacts.